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L5
DIFFRACTION
OBJECTIVES
Aims
From this chapter you should gain an understanding of the process of diffraction and its role in
producing distinctive interference patterns. You should also aim to understand how the effects of
diffraction can affect and limit the formation of images. As the classical example of diffraction,
you should be able to describe and explain the Rayleigh criterion and apply it to simple examples.
Minimum learning goals
1.
Explain, interpret and use the terms:
diffraction, diffraction pattern, Fresnel diffraction, Fraunhofer diffraction, angular
resolution, Rayleigh criterion, diffraction envelope, principal maximum, secondary maxima,
double slit, diffraction grating.
2.
Describe qualitatively the diffraction patterns produced in monochromatic light by single
slits, rectangular apertures, circular apertures, double slits and diffraction gratings.
3.
Describe the interference pattern produced by monochromatic light and a grating in terms of
the interference pattern produced by a set of line sources modulated by a diffraction
envelope.
4.
State and apply the formulas for the angular widths of the central maxima in the Fraunhofer
diffraction patterns of a single slit and a circular hole.
5.
State the Rayleigh criterion, explain its purpose and apply it to simple examples.
6.
Describe how wavelength, slit width, slit spacing and number of slits affect the Fraunhofer
diffraction patterns produced by multiple slits and gratings.
7.
State and apply the formula for the angular positions of maxima in the Fraunhofer
diffraction patterns produced by multiple slits and gratings.
PRE-LECTURE
5-1 SHADOWS
In the ray model we suppose that when light travels through a homogeneous medium it moves
along straight lines. That observation is often called the law of rectilinear propagation. The
existence of shadows is good evidence for the ray model of light. When light from a small (or
'point') source goes past the edges of an opaque object it keeps going in a straight line, leaving the
space behind the object dark (figure 5.1).
Figure 5.1. Straight line propagation of light
A small source of light produces sharp shadows.
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When the light reaches some other surface the boundary between light and dark is quite
sharp. (On the other hand if the light comes from an extended source the shadow is not so sharp there is a region of partial shadow surrounding the total shadow.)
It was not until the about the beginning of the nineteenth century that scientists noticed that
shadows are not really perfectly sharp. Looked at on a small enough scale the edge of a shadow is
not just fuzzy, as you might expect for an extended source of light, but there are also light and
dark striations or fringes around the edge of the shadow. Even more remarkable is the slightly
later discovery that there is always a tiny bright spot right in the middle of the shadow cast by a
circular object (figure 5.2). The fringes and the bright spot cannot be understood in terms of the
ray model - the explanation lies in the wave model. According to the wave theory, the fringes are
formed by the diffraction or bending of light waves around the edges of objects and the
subsequent interference of the diffracted waves. The diffraction of water waves at a hole in a
barrier was shown in the video lecture on interference (L4) and it is sketched in figure 4.4.
One of the effects of diffraction is the production of interference or fringe patterns.
Although these patterns are actually interference patterns in the same sense as those we have
already discussed, when they are produced by the bending of light around obstacles or apertures
(holes) they are called diffraction patterns.
Figure 5.2. Shadow of a small circular object
The edges of the shadow contain fringes and there is a small bright spot in the middle.
Young's twin slits experiment shows that light does not always travel along straight lines.
You can see that it must bend somewhere by looking at figure 5.3. Since there is light at the
middle of the screen but no straight through path from the source to the screen, the light which
gets there must somehow be going around corners. What must be happening is that after the light
reaches the first slit it then spreads out so that some of it reaches the other two slits. Then, having
passed through those slits it spreads out again in many directions until it reaches the screen. This
behaviour is typical of waves, but not of particles.
Light
source
First
slit
Pair of
slits
Screen
Figure 5.3. Arrangement for Young's experiment
Light cannot be travelling in a straight line all the way from the source to the screen.
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The connection between waves and diffraction is much more noticeable for sound than it is
for light. The observation that sound easily travels around corners is strong evidence for the wave
nature of sound.
TEXT & LECTURE
5-2 HUYGENS' CONSTRUCTION
A way of describing how diffraction occurs was invented by Christian Huygens (1629-1695) in
about 1679 and was modified much later into the form we now use by Augustin Fresnel (1788 1827). Huygens' construction is a method for locating the new position of a wave front. Starting
from a known wavefront, we imagine each point on the wavefront to be a new source of
secondary wavelets. The wavelet from each point then spreads out as a sphere (which appears as
a circle in two-dimensional diagrams). After a certain time the new position of the original
wavefront is defined by the boundary or envelope of all the secondary wavelets. Huygens
construction for a plane wave going through a slit is shown in figure 5.4; after it has passed
through the hole, the wavefront is no longer plane, but has curved edges. The result of Huygens'
construction is significantly different from the ray model in that it shows light waves spreading
into the region of the geometrical shadow. You should notice that opposite the slit the wavefront
is still plane; it bends only at the edges. This bending effect is noticeable only on a scale
comparable with the wavelength - for a very wide hole comparatively little of the wavefront bends
around the edges.
1
2
3
4
5
Figure 5.4. Huygens' construction
Five stages in the progress of a wavefront through a slit. The diagrams show the construction for three equal
time intervals after a plane wave reaches the slit, travelling left to right. At any stage the new wavefront is the
boundary of all the wavelets used in the construction.
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Although the Huygens construction 'explains' how new wavefronts are formed, it does not
predict the wave's amplitude; other methods are needed for that. The construction does however
contain a clue about the strength of the waves. If you look at the straight-through wave in figure
5.4 you will see that there are many wavelets, but on the sides relatively few wavelets appear,
which would seem to suggest, correctly, that the diffracted wave is weaker on the sides.
5-3
PRODUCING DIFFRACTION PATTERNS
Opaque screen
with aperture in the middle
Viewing screen
Light s ource
Diffraction pattern
Figure 5.5. Producing a diffraction pattern
(Not to scale)
Details of the distribution of light over the diffraction pattern depend on the distances of
both the source and the screen from the diffracting aperture. The general situation (figure 5.5),
known as Fresnel diffraction, can be mathematically very complex. However, calculations are
greatly simplified if both the source and the screen are at very large distances from the aperture
(i.e. if those distances are much greater than the diameter of the aperture). We will deal
quantitatively only with this situation, which is known as Fraunhofer diffraction.
Fraunhofer conditions can be achieved in practice by using two lenses (figure 5.6). The first
lens ensures that the wavefronts arriving at the aperture will be plane (with parallel rays) and the
second lens brings beams of diffracted light together to form an interference pattern on the screen.
B
A
Light s ource
Viewing
s creen
Aperture
fA
Screen
fB
Figure 5.6. Producing Fraunhofer diffraction
Lens A ensures that the wavefronts which arrive at the aperture are plane. In order to see what the diffraction
pattern at infinity would be like, lens B is used to produce its image on the screen.
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5-4 DIFFRACTION AT A SINGLE SLIT
You will recall that light consists of a superposition of many elementary waves, with a wide range
of frequencies. In the simple theory of diffraction we deal with one frequency at a time. The
diffraction of a complex beam of light is then described in terms of what happens to each of the
component waves with different frequencies.
The simplest case of Fraunhofer diffraction is that for a long narrow slit in an opaque
screen. The interference pattern consists of a set of light and dark parallel fringes (figure 5.7).
Figure 5.7. Single slit diffraction pattern
Figure 5.8 shows how the diffraction by a slit is studied using Fraunhofer conditions. The
first lens ensures that the wavefronts arriving at the aperture will be plane and the second lens
focusses the light onto the screen.
Diverging
wavefronts
Plane
wavefronts
Complex wavefronts
not shown
O
P0
Source
P
Lens A
Slit
Screen
Lens B
Figure 5.8. Fraunhofer diffraction by a single slit
After passing through the slit light waves spread out in all directions. The second lens brings parallel groups of
rays to a focus on the screen. If there were no diffraction there would be only one such focus (P ) for each
0
source point O.
The wavefronts arriving at the slit are plane, but because of diffraction, the wavefronts on
the other side will not be plane. Light arriving at any point in the aperture has a fixed phase
relationship to light from the same part of the source arriving at any other point in the aperture.
The simplest case is to imagine an elementary plane wavefront arriving parallel to the slit as
shown in figure 5.8. Since all points on a wavefront have the same phase we can imagine the
aperture filled with many tiny coherent sources. When the light from all of these sources comes
together at various points on the screen an interference pattern will be seen. The brightness at any
point will depend on the phase differences among all the secondary waves arriving there and those
phase differences will depend on the optical paths travelled by the different waves. Path
differences can be calculated using rays which leave the slit parallel to each other, so that they
would eventually meet at infinity were it not for the presence of the second lens. Since the lens
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itself introduces no additional optical path difference, calculations can be done on the assumption
that the rays meet at infinity.
The following argument, given in the video lecture, shows how to work out the condition for a
minimum in the interference pattern. Consider rays coming from various coherent points spread across
the slit (figure 5.9).
P0
a
Lens B
Slit
Screen
Figure 5.9. Formation of the central bright fringe
All these rays have the same optical path length.
All the rays parallel to the axis will be focussed at P0. Although the geometrical path lengths of
the rays are obviously different, the optical paths from different points across the slit to the point P 0 are
all equal. That is so because the longer paths outside the lens are compensated by shorter optical paths
of the rays within the lens. Rays from the outside of the slit go through more air but less glass than rays
near the middle of the lens. So all the light arriving at P 0 is in phase, giving a bright region there.
Enlarged
view

P1

a

2
P
0
a
D
D
Figure 5.10. Condition for a minimum - Fraunhofer diffraction
For other points on the screen the phases for light arriving along a parallel group of rays are all
different, but it is fairly straightforward to work out where there will be complete cancellation. Pick
two parallel rays, one leaving from the top of the slit and the other from a point halfway across it (figure
5.10). Both rays leave the slit at an angle to the axis. If the angle is chosen so that the path
difference between these two rays is some odd multiple of half a wavelength (D = /2, 3/2,.. etc) they
will interfere to produce a minimum in the irradiance at the screen.
Any other pair of rays half a slit width apart and leaving at angle  will also interfere to give a
minimum. We can choose similar pairs of rays until the whole aperture has been covered. Since all
pairs produce a minimum there will be a dark region at P . From the small triangle in the diagram it
1
can be seen that the path difference for every pair of rays is equal to (a/2)sin. Finally equate this
expression with the value of D (above) needed for a minimum.
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The condition for an interference minimum is that
sin =
m
a
for m = ±1, ±2, ±3 ... .
... (5.1)
Principal maximum
Irradiance
Secondary maxima
-20
-10
0
10
20
Angular pos ition/ degrees
Figure 5.11. Single slit diffraction pattern - Fraunhofer conditions
A plot of the intensity of the light against position on the screen is shown in figure 5.11.
Several features of the diffraction pattern are worth noting.
•
The angle between the central peak and the first minimum (m = 1) is given by

sin = a .
•
•
•
So the widest diffraction patterns are produced by the narrowest slits. If the slit is only one
wavelength wide the angular position of the first minimum would be 90° giving an angular
separation of 180° between the zero intensity positions - so the diffracted light spreads out
in all directions. On the other hand if the slit is wide, the angle of the first minimum is
small and diffraction effects may be hard to see.
The principal maximum is roughly twice the width of the secondary maxima. You can see
that by putting sin≈  in equation 5.1 and comparing the change in angular position going
from m = 1 to m = -1 with the change in going from m = 1 to m = 2, for example.
The width of the diffraction pattern depends on wavelength. The pattern for red light is
wider than that for blue light.
The intensities of the secondary maxima are very much less than the intensity of the
principal maximum.
5-5 DIFFRACTION BY A CIRCULAR APERTURE
In practice the most important example of diffraction produced by a single aperture is that for a
circular hole. Most optical instruments have circular apertures or lenses that act as circular
apertures, so whether we wish it or not we will get diffraction effects.
For Fraunhofer diffraction at a circular hole, the plot of intensity against position on the
screen (figure 5.12) is similar in its general shape to that for a single rectangular slit. However it
differs from the single slit pattern in the following ways.
•
The diffraction pattern is a circular patch of light (called the Airy disc) surrounded by rings
of light.
•
The angle between the principal maximum and the first minimum is about 20% greater than
that for a slit of the same width:

sin  1.22 a .. (5.2)
•
The spacings between adjacent minima are not as uniform as those for the slit.
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•
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The intensities of the secondary maxima are smaller.
Irradiance
min
0
Angular pos ition 
Figure 5.12. Diffraction pattern for a circular aperture
At a large distance from the hole (Fraunhofer conditions) the first minimum in the irradiance
occurs at an angular position sin 
= 1.22 /D. The secondary maxima are less bright than
min
those for a slit.
Example
It is important to realise the size of the diffraction pattern for an average sort of lens, say 50 mm in
diameter, which acts as a circular aperture whenever it is used to form images. Consider a typical
visible wavelength of 500 nm and calculate the angle between the central maximum and the first
minimum.
sin
=

l.2 D
-9
1.2  500  10 m
-5
= 1.2 10 .
-3
50  10 m
-5
Since is small, sin  ≈ and= 1.2  10 rad or0.0007°.

 
5-6 YOUNG'S DOUBLE SLIT
Young's classic interference experiment with the two slits has already been described (§§4-2, 4-4)
and quoted as evidence for the diffraction of light (§5-2). Detailed descriptions of the two-slit
interference pattern involve the width of a slit, a, and the separation between the slits, d (figure
5.13). Note that d > a.
d
Figure 5.13
Dimensions of a double
slit
a
a
A simple analysis of the experiment assumed that both the sources were very narrow - much
less than a wavelength wide (a << ). In practice however real slits are not that narrow and the
interference pattern depends on the width of the slits as well as their separation. The kind of
interference pattern produced is illustrated in figures 5.14 and 5.15.
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Figure 5.14. Diffraction pattern for a pair of slits
For Fraunhofer conditions the plot of irradiance against angular position on the screen
shows a pattern of almost equally spaced fringes with different brightnesses (figure 5.15). The
maximum irradiance occurs in the central bright fringe, at position  = 0. Notice how the fringes
occur in groups, with the brightest fringes in the middle group.
Irradiance
Angular pos ition
Figure 5.15. A typical two-slit interference pattern - Fraunhofer conditions
Figure 5.16 shows a comparison between the interference patterns for one and two slits.
You may recall from chapter L1 that the maximum intensity for two identical coherent sources
should be four times (not twice) the intensity for one source. That result posed a puzzle about
conservation of energy. Energy or power is distributed differently in the two cases. Although the
peak of the two-slit curve is four times higher than that of the single-slit curve, there are places
where there is no light at all. The total power is represented by the areas under the graphs and, as
expected from conservation of energy, the area under the two-slit curve is twice the area under the
one-slit curve.
One slit
a = 5
Irradiance
-15
-10
-5
0
5
10
15
Two slits
a = 5
d = 20 
-15
-10
-5
0
5
Angu lar pos ition /degrees
10
15
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Figure 5.16.
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Irradiance and total power for one and two slits
Each slit is 5 wavelengths wide. These graphs are plotted on the same scale. Two identical slits give a
maximum irradiance equal to 4 times the maximum irradiance from one slit. The total power, represented by
the area under the curve, is only twice as large.
The interference pattern in Young's experiment can be described as a combination of the
interference pattern for two very narrow slits and the pattern produced by one slit of finite width.
When the mathematical forms of these two patterns are multiplied together we get the shape of
the observed two-slit pattern. The spacing of the fringes is determined by the separation of the
slits but their brightness is influenced by the width of a slit. Figure 5.17 shows how the
interference pattern of two slits with zero width is modulated by the pattern of a single slit.
Ideal two slit
interference
pattern.
Slit width = 0.

Irradiance
Slit width = a
Single s lit
diffraction pattern
=
Double slit pattern
Mis sing
order
Separation
d = 5a
Angular pos ition
Figure 5.17. Young's slits pattern as a combination of two patterns
The ideal two-slit interference pattern (top graph) multiplied by the diffraction pattern for one slit (middle)
yields the double slit diffraction pattern (bottom). The vertical scales in these graphs are not the same.
Since the description of the two-slit interference is just a combination of two simpler cases
that we have already considered, it is to be expected that the results we got earlier should carry
over. The locations of the interference maxima are still given by equation 4.3. For the
m
mth order maximum:
sin = d ; m = 0, ±1, ±2, ... .
... (5.3)
And the places where there would be a zero in the single slit pattern are still dark; for a non-zero
integer value of n , there is a minimum when
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n
a ; n = ±1, ±2, ... .
sin =
... (5.4)
If you look again at figure 5.17 you can see that in the place where the fifth order fringe
(m = 5) ought to be there is nothing (remember to count the central fringe as number 0). That
happened because the condition for the first zero (n = 1) in the single slit pattern coincided with
the position of the fifth bright fringe in the two-slit pattern. By comparing the two conditions
(equations 5.3 and 5.4) you can see that the missing fringe was caused by the fact that the slit
spacing in the example was chosen to be exactly five times the slit width. Other missing orders
may be caused by similar coincidences.
Figure 5.18 shows how the single slit pattern forms an envelope to the double slit pattern
for some different values of the slit width but the same slit spacing. You could say that the twoslit interference pattern has to be squeezed inside the single slit pattern (figure 5.18).
Diffraction envelope
a = 
Irradiance
-15
-10
-5
0
5
10
15
Diffraction envelope
a = 5 
-15
-10
-5
0
5
10
15
Diffraction envelope
a = 10 
-15
-10
-5
0
5
10
15
Angular pos ition/degrees
Figure 5.18 Effect of slit width on the fringe pattern
Although widening the slits lets more light through and increases the overall brightness, these graphs have been
normalised to have the same maximum irradiance. Making both slits wider reduces the width of the diffraction
envelope and the relative brightness of the outer fringes.
5-7 RESOLUTION OF IMAGES
The theory of diffraction shows that a point source of light or an object point cannot possibly
produce a perfect point image even in an optical system which is free of aberrations. The wave
L5: Diffraction
nature of light places a fundamental limitation on the quality of an image. Resolving power is a
somewhat loose term which refers to the ability of an optical system to distinguish fine detail in
an image. It can be illustrated by considering how one might distinguish between the images of
two stars in a telescope. Stars are so far away that they can be considered as point objects, and
the fact that they often appear to have finite sizes is due to diffraction. Although a refracting
telescope normally contains several lenses, the concept of resolving power can be understood by
representing the system as one lens (the objective lens), and an aperture as shown in figure 5.19.
The centres of the real images of the stars are formed in the focal plane of the objective lens and
are separated by an angle  which is equal to the angle subtended by the two stars at the objective.
(The images are viewed using the telescope's eyepiece.)
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Irradiance
Two distant
point s ources


Focal
plane
Lens and
diffracting
aperture
Figure 5.19. Resolving the images of two point sources
The sources are incoherent so the images are two separate diffraction patterns.
(Note: the figure is not a ray tracing diagram.)
If the angular separation  is so small that the images are effectively on top of each other, it
will not be possible to recognise that there are two separate images. The angular resolution of
an instrument is the smallest angular separation that can be distinguished. Although the decision
about the actual value of an angular resolution may depend on the skill of the observer, the
relative brightness of the sources and other factors, it is useful to have a generally agreed
definition of when the two images of two incoherent point sources can be resolved. The criterion
generally used was proposed by Lord Rayleigh (1821-1894): the two point sources are just
resolved if the central maximum of one image coincides with the first minimum of the other.
For a circular aperture and Fraunhofer diffraction, you can see from equation 5.2 that the
Rayleigh criterion is satisfied when the angular separation of the two point sources has the value
min given by
1.22
sinmin =
a
where a is the diameter of the aperture. In practice, since the angles involved here are always
very small we can put sin = and since 1.22 is near enough to 1, the Rayleigh criterion for a
circular aperture is that

angular resolution = a
.
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 = 2min
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Rayleigh criterion
sin min = / d
Figure 5.20. Rayleigh criterion for two point sources
The pair of images on the left is easily resolved. The two images on the right are just resolved according to
Rayleigh's criterion.
It follows that to improve resolving power one can use a shorter wavelength, which is
usually not possible, or a larger lens which gives a larger aperture. That is one of the reasons that
modern astronomers need large telescopes.
The discussion above applies directly to telescopes and other optical instruments which are
used to look at incoherent sources. However the details do not necessarily carry over to all kinds
of microscopy, because the illumination of adjacent parts of a specimen may be at least partially
coherent. The analysis of such cases is more difficult, but the general idea that resolution can be
improved by using a larger aperture or a shorter wavelength remains valid.
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5-8 DIFFRACTION GRATINGS
1 slit
2 slits
Irradiance
4 slits
8 slits
Angular pos ition
Figure 5.21. Multiple slit fringe patterns
These graphs have been normalised to have the same value of the central maximum in order to emphasise the
2
effect on the fringe width. In fact using N slits instead of one slit makes the central maximum N times
brighter. In the examples above the separation of the slits is 5 times the slit width so that the fifth order fringe
is missing.
Now that you have seen how a double slit pattern is formed, it is interesting to ask what
happens if the number of equally spaced slits in a Young's experiment is increased. There are two
main effects. Firstly, and fairly obviously, the whole diffraction pattern becomes brighter
because more light gets through. The second effect is more surprising - the bright fringes get
sharper! Although the positions of maximum irradiance remain unaltered the width of each bright
fringe decreases as the number of slits increases. (A third, but less noticeable, effect is that some
new weak fringes appear in between the principal maxima.)
The reason that the principal fringes stay in the same locations is that the conditions for
maximum irradiance are still essentially the same - the optical path difference from adjacent slits
to the point on the screen needs to be a whole number of wavelengths and that condition does not
depend on how many pairs of slits you have. On the other hand, if there are many slits then the
condition that all possible adjacent pairs of slits must give the same path difference is much
tighter. Consider a point on the screen just a little bit away from a peak. The corresponding
points on the wavefront at two adjacent slits do not have the exactly right phase difference for a
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84
maximum and the phases of the waves from other slits will be even more out of step. The more
slits you have the worse the matching of the phases will be.
Although the fringes are much sharper their positions are still described by the same
Young's slits equation (5.3).
The sharpening of the fringes is exploited in the diffraction grating, a device which
consists of a very large number of narrow, uniformly spaced, parallel slits (typically 1200 slits per
millimetre). There are two kinds of grating. A transmission grating is made by cutting grooves
in a material such as glass; the grooves are effectively opaque strips and the unruled portions are
the slits. A reflection grating works by reflecting light from many parallel mirror-like strips.
Since the fringe spacing depends on wavelength and because the bright fringes are very
sharply defined, the diffraction grating can be used in a spectroscope or spectrograph to spread a
beam of light into components with different frequencies. The grating is far superior to the prism
because it can be made to give a much greater angular separation of the spectrum. If the slit
separation is very small (say 10-6 m) and if there are many slits (say 106), then a line spectrum
will have very sharp, clearly-separated interference maxima.
THINGS TO DO
•
The grooves on an LP record or (better) a compact disc can function as a reflection
diffraction grating. Look at various sources of light reflected from the surface of a disc. How
many orders of the diffraction pattern can you see? Can you estimate the spacing of the grooves?
•
You can use a piece of finely woven cloth as a kind of two-dimensional transmission
grating. Look through the cloth at a mercury or sodium street light.
QUESTIONS
Exercises
Q5.1 Estimate the width of the central maximum of a single slit diffraction pattern which appears on a screen 1.0 m
away from a slit of width 0.10 mm, illuminated with light of  = 500 nm.
Q5.2
?
2a
a
Screen
Light s ource
Using the result that the angle between the centre and the first minimum of a single long slit diffraction

pattern is sin = a , can you predict what the diffraction pattern of the rectangular aperture in the figure will
look like?
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85
Q5.3 Estimate the angular width of the central maximum of the diffraction pattern produced by a circular aperture
2.0 mm in diameter. Take = 500 nm.
Q5.4 The two headlights of a distant approaching car are l m apart. Make a rough estimate of the distance at which
the eye can resolve them. Take a pupil diameter of 5 mm and use a typical visible wavelength, say  = 500
nm.
Q5.5 A grating with 400 grooves per millimetre is used to examine the spectrum of a source of light.
a)
b)
Calculate the angle between the first order maxima for red light with  = 700 nm and violet light with
 = 400 nm.
Do the same for the two components of the sodium-D line which have wavelengths 589.0 nm and
589.6 nm.
Discussion questions
Q5.6 Can you explain why the centre of the diffraction pattern for a circular obstacle always contains a bright spot?
Q5.7 A grating will not be of much use for producing a clear spectrum if the first order fringes of some part of the
spectrum overlap the second order fringes of some other part. Discuss. How should the grating be constructed
in order to avoid the problem?
Q5.8 Why is diffraction more noticeable for sound than for light?
Q5.9 Light waves do not bend noticeably around buildings, but radio waves which are also electromagnetic waves
do diffract around buildings. Discuss.
Q5.10 What do you think of the claim that you can't see an interference pattern in a Young's experiment if the
separation of the slits is less than half a wavelength?
Q5.11 What would you see in a Young's two-slit experiment with white light?
Q5.12 Why is a grating better than just two slits in an experiment to measure wavelength?
APPENDIX
MATHEMATICAL DESCRIPTION OF THE DIFFRACTION PATTERNS
The shape of the Fraunhofer intensity distribution curve for a grating with N slits is described by the
equation:
sin 2 sin(N) 2
I = A
    sin 
πa
where
 =
sin

which is half the phase difference between one edge and the middle of a slit
πd
and
 =
sin

which is half the phase difference between corresponding points on adjacent slits. A is a constant which
represents the amplitude of the incoming wave - assumed to be the same at all slits.
The single slit diffraction pattern (N = 1) and the Young's double slit case (N = 2) are also
described by the same equation.
sin 2
The envelope (single slit pattern) is described by the term 
while the idealised pattern
  
sin(N) 2
for N slits of zero width is given by the term 
.
 sin 
The results for the zeros in the diffraction envelope (equation 5.1) and the condition for the
maxima (equation 5.3) can be obtained from the equation above.